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FROG DISSECTION – LAB PROCEDURE
PRE-LAB DISCUSSION
Frogs belong to the class Amphibia. Amphibians have adaptations for living in terrestrial as well as aquatic
environments. Frogs are among the most commonly studied organism in biology. Although many differences exist
between humans and frogs, the basic body plans are similar. Humans and frogs both belong to the phylum Chordata.
By studying the anatomy of the frog, you will be better able to understand your own body.
In this investigation, you will observe the behavior of a live frog. You will examine the external features of a
frog and identify parts of its external anatomy. In addition, you will dissect a preserved frog to observe its internal
anatomy.
Before you begin working with the frogs, assign the following jobs to each person in your group:
Surgeon = handles majority of cutting & location of parts
Assistant Surgeon = in charge of tools, assisting in handling frog & locating parts
Reader = directs the surgeon and assistant surgeon throughout dissection
Recorder = will discuss questions/diagrams with the group & write them in the lab packet
* ALL MEMBERS OF YOUR GROUP ARE RESPONSIBLE FOR KNOWING ALL STRUCTURES!*
Answer the Pre-Lab questions in your frog dissection packet before continuing.
PROCEDURE:
Part A. External Anatomy of the Frog
1. Obtain a preserved frog. Rinse the frog with water to remove excess preservative. Dry the frog with paper
towels and place it in a dissecting tray. [CAUTION: The preservative used on the frog can irritate your
skin. Avoid touching your eyes while working with the frog.]
2. Identify the dorsal (the back side) and ventral (the belly side) surfaces of the frog. Identify the anterior
(front) and posterior (back) ends of the frog. Answer question #4 in the lab packet.
3. Locate the forelegs and hind legs. Each foreleg, or arm, is divided into four regions: upper arm, forearm,
wrist and hand. Each hind leg also has four regions: thigh, lower leg, ankle and foot. Identify the parts of
forelegs and hind legs. Examine the hands and feet of the frog. If the hands have enlarged/bulging
thumbs, the frog is a male. Answer questions #5-7 in the lab packet.
4. Locate the two large, protruding eyes. Lift the outer eyelid using a probe or dissecting needle. Beneath
the outer lid is an inner lid called the nictitating membrane – it acts like a third eyelid. Answer question
#8 in the lab packet.
5. Posterior to each eye is a circular region of tightly stretched skin. This region is the tympanum (tympanic
membrane), otherwise known as the eardrum. Locate the tympanic membranes on both sides of the
nostrils.
6. Anterior to the eyes, locate the two openings called external nares (singular = naris), or nostrils.
7. In the appropriate place in the lab packet, label the diagram of the external anatomy of the frog using the
following terms:
Anterior end, posterior end, dorsal, ventral, forelimb, hindlimb, hand, foot, tympanic
membrane, external nares, eye, nictitating membrane and mouth.
* CHECK POINT!! IDENTIFY STRUCTURES TO TEACHER FOR A STAMP!!
8. Hold the frog firmly in the dissecting tray. Using scissors make a cut (approximately one inch) at each of
the hinged points of the jaw, as shown in FIGURE 1. [CAUTION: To avoid injury, cut in a direction away
from your hands and body.] Open the mouth as much as possible (almost to 180°). Under the running
water, rinse away excess preservative.
9. The tongue is the most noticeable structure in the mouth. Observe where the tongue is attached and note
the two projections at the free end. Answer question #9 in the lab packet.
10. At the back of the mouth, locate the large horizontal opening – this is the gullet opening (leads to the
esophagus). In front of the gullet opening, find a vertical slit called the glottis.
11. Look for two openings on the backsides of the floor of the mouth. These are the opening to the vocal sacs.
They are present in male frogs but not in female frogs.
12. Examine the roof of the mouth. Near the front center of the roof of the mouth are two small bumps.
These bumps are the vomerine teeth. On either side of the vomerine teeth, observe two large bulges.
These bulges are the eye sockets. Run your fingers along the top jaw. The teeth you feel are the maxillary
teeth. The openings of the Eustachian tubes are on either side near the back of the mouth. Insert a probe
or dissecting needles into an opening of one Eustachian tube. Note where the probe stops.
13. In the appropriate place in the lab packet, label the diagram of the frog’s mouth using the following terms:
Vomerine teeth, maxillary teeth, internal nares, eye sockets, openings to Eustachian tubes,
tongue, gullet opening, glottis and opening to vocal sacs
* CHECK POINT!! IDENTIFY STRUCTURES TO TEACHER FOR A STAMP!!
Part B. Internal Anatomy of the Frog.
1. Place your preserved frog in the dissecting tray with the ventral surface up. With dissecting pins securely
pin the frog’s feet and hands to the bottom of the dissecting tray as shown in FIGURE 2. Angle the pins
away from the body of the frog so that they will not interfere with you dissecting.
2. Carefully lift up/pinch the loose skin of the abdomen. Carefully insert the tip of a pair of scissors beneath
the skin. [CAUTION: To avoid cutting yourself, cut in the direction away from your hands and body.] Cut
the skin along line AB as shown in FIGURE 2. Continue cutting the skin along the lines CD and EF. Be sure
to cut the skin all the way to the dorsal (back side) of the frog.
3. With your fingers, carefully separate the skin from the underlying muscles. Open the flaps of skin as far
back as possible and pin them to the bottom of the dissecting tray. Angle the pins away from the body of
the frog so that they will not interfere with your dissection. Notice the blood vessels branching throughout
the inner lining of the skin. Observe the abdominal and pectoral muscles. Note the direction of the
muscle fibers.
4. Carefully lift the abdominal muscles and cut a second AB incision [NOTE: Keep the cut through the muscles
shallow so as not to damage underlying organs. Do not cut straight down into the frog – the bottom blade
of the scissors should be against the muscle you are cutting]. As the incision is made in the chest, or
pectoral area, you will need to cut through bone. This bone is part of the pectoral girdle [NOTE: Use extra
force with the scissors when cutting through the bone]. Be careful not to damage any of the internal
organs below the bone. Make cuts CD and EF through the abdominal muscle.
5. Remove the pins holding the skin in place. Stretch the abdominal opening as much as possible. At this
time the hands and feet of the frog may need to be re-pinned.
6. Study the positions of the exposed organs. Notice that most of the organs are held in place by very thin,
transparent tissues called mesenteries.
7. If the frog is a mature female, the most obvious organs will be the ovaries. The ovaries are black sacs
swollen with tiny black-and-white eggs. Carefully life the ovaries from the body cavity, cut the
attachments with scissors and remove the ovaries from the frog [NOTE: Be careful not to rupture the
ovaries with the scissors. If the ovaries are ruptured, they can spill out a mess of eggs].
8. The large brown organ in the upper part of the abdominal cavity is the liver. Answer question #10 in the
lab packet.
9. With your fingers or a dissecting needle/probe, life and separate the lobes of the liver upwards. Behind
the middle lobe, look for a small, greenish gland – this gland is the gallbladder.
10. With scissors, carefully remove the liver and gallbladder from the body. The remaining organs of the
digestive system are easier to see with the liver removed.
Digestive System
11. Locate the esophagus, which is a light colored tube leading from the mouth and connecting to the upper
part of the muscular stomach. Notice the shape of the stomach. Look for a constriction at the lowest part
of the stomach – this is called the pylorus. The pylorus leads into the long, coiled small intestine.
12. Pull the loops of small intestine away from the body. Notice the mesentery that holds the intestines in
place. Inside the first loop of the small intestine near the stomach, locate a thin, white organ called the
pancreas. Also in the intestinal mesentery, locate a brown, bean-shaped organ called the spleen. [NOTE:
The spleen is an organ of the circulatory system.] Answer question #11 in the lab packet.
13. The small intestine ends in a large bag-shaped organ – the large intestine. The last organ of the digestive
system is the cloaca, a saclike organ at the end of the large intestine. Undigested food leaves the frog’s
body through an opening called the anus.
14. With scissors, cut the esophagus near the stomach. Cut through the large intestine just above the cloaca.
With your fingers, carefully remove the digestive system from the body.
15. Stretch out the digestive system on the dissecting tray by carefully tearing the mesentery. With scissors,
cut open the stomach along its outside curve. Open the stomach and examine its structure and contents.
Answer questions #12-13 in the lab packet. Set the digestive system, liver and ovaries off to the side in
the dissecting tray.
16. In the appropriate place in the lab packet, label the diagram of the frog’s digestive system and related
organs using the following terms:
Esophagus, stomach, pylorus, small intestine, large intestine, cloaca, liver, gallbladder,
pancreas, mesentery, anus and spleen
* CHECK POINT!! IDENTIFY STRUCTURES TO TEACHER FOR A STAMP!!
Urogenital System
17. The reproductive system and urinary system of the frog are closely connected and can be studied as the
combined urogenital system. The two kidneys are brown organs located on the dorsal posterior wall of the
abdominal cavity. The kidneys lie on either side of the backbone. [NOTE: The kidneys may be covered
with a thin membrane. If so, carefully tear open the membrane with the point of the dissecting needle.]
The yellow, fingerlike lobes attached to the kidneys are fat bodies –remove them by cutting them out and
put them off to the side. A small, twisted tube called the ureter leads from each kidney into the saclike
urinary bladder. The bladder is connected to the cloaca.
18. Locate the reproductive organs of the frog. If your frog is a male, it possesses testes, tiny white or yellow
oval organs found on the ventral surface of the kidneys.
19. If your frog is female, it possessed egg-filled ovaries that were removed previously. If your frog is an
immature female, the pale oval ovaries are located ventral to the kidneys. Leading from each ovary is a
long, extremely coiled tube called the oviduct. The oviduct extends along the side of the body cavity. The
oviduct eventually joins the cloaca.
20. In the appropriate place in the lab packet, label the diagrams of the male and female urogenital systems
(you must label both even though you have only one frog!):
Kidney (both diagrams), fat bodies (both diagrams), ureter (both diagrams), urinary
bladder (both diagrams), cloaca (both diagrams), testes, ovary filled with eggs and
oviduct
* CHECK POINT!! IDENTIFY STRUCTURES TO TEACHER FOR A STAMP!!
Respiratory System
21. Locate the two lungs. They are small, spongy brown sacs that lie to the right and left of the heart. Look
for the bronchial tubes that extend from the anterior part of the lungs and join with the trachea, or
windpipe.
22. Insert a dropper or disposable pipette into the glottis of the frog. Pump air into the lungs and observe
what happens. If this does not work, carefully remove one lung and pump air into it. Answer question
#14 in the lab packet.
23. With scissors, carefully remove the lungs from the frog’s body. Set them off to the side in your dissecting
tray.
Circulatory System
24. Locate the heart. The heart is encased in a membranous sac called the pericardium. With the tip of the
scissors, carefully cut open the pericardium.
25. Note the vessels attached to the heart. The large artery on the ventral surface of the heart is the coronary
artery. [NOTE: If the frog has been injected with red and blue latex paint, the veins and arteries will be
obvious.]
26. Carefully cut the blood vessels leading to and from the heart. Remove the heart from the frog. Place the
heart in the dissecting tray with the dorsal surface facing up. Identify the right and left atria and the
ventricle. Touch and slightly squeeze the walls of the two atria and the ventricle. Answer question #15
in the lab packet.
27. Observe the dorsal surface of the heart. Locate the thin-walled triangular sac called the sinus venosus.
Locate the two veins leading from the top and the one vein leading from the bottom of the sinus venosus.
28. With scissor, cut the heart into anterior and posterior halves. Note the thickness of the walls and the types
of heart chambers. [CAUTION: Be careful when cutting – always cut in a direction away from your hands
and body.]
29. In the appropriate place in the lab packet, label the diagram of the frog’s heart using the following terms:
Right atrium (both diagrams), left atrium (both diagrams), ventricle (both diagrams),
coronary artery and sinus venosus
* CHECK POINT!! IDENTIFY STRUCTURES TO TEACHER FOR A STAMP!!
Muscular System [if time permits]
30. Remove the pins from the frog’s feet and hands.
31. Cut the skin completely around the upper thigh of one leg, as if cutting off the leg of a pair of pants.
Carefully pull the skin downward to the foot. Expose the thigh muscles, the knee and the calf muscles.
32. Move the lower leg up and down to simulate the leg movement during a jump. Observe the various leg
muscles involved in the leg movement.
FOR CLEAN-UP, PLEASE BE SURE ALL OF THE FOLLOWING ARE COMPLETED:
BE SURE ALL PARTS OF THE FROG ARE PROPERLY DISPOSED AND DO NOT GO DOWN THE SINK!!
Thoroughly WASH, DRY and PUT AWAY your dissecting tray, tool layout sheet and tools.
Wipe down and clean your lab table.
Wash your hands with soap and water.
WITH YOUR GROUP, DISCUSS AND ANSWER THE REMAINING ANALYSIS/CONCLUSION AND CRITICAL
THINKING QUESTIONS. STUDY YOUR QUESTIONS & DIAGRAMS FOR A QUIZ ON FROG DISSECTION!!